Thin film resonator, or TFR, technology, has received much interest over the last several years. The thin film resonator technology makes possible a class of thin film microwave acoustic devices that are truly compatible with active semiconductor circuitry. The small size of the thin film resonator is compatible with semiconductor technology, and the thin film resonator can be integrated with semiconductor devices onto a common substrate.
To operate in the fundamental mode at VHF to microwave frequency ranges, a resonator must have a thickness in the range of tens of microns to less than one micron. Devices of such thicknesses are very fragile and easily damaged, and require some form of external support during and after manufacture for any practical application. This requirement has given rise to the development of etching techniques which provide for the placing of the device on a silicon substrate, with a cavity etched into the silicon underneath the device to allow free movement of the device. This permits the edges of the device to be supported by the silicon substrate.
The basic thin film resonator technology uses DC magnetron sputtered highly-oriented thin films of dielectric material, preferably aluminum nitride (AlN) or zinc oxide (ZnO). The dielectric film is sandwiched between a pair of conductive electrodes, typically thin film aluminum electrodes, and the electrodes serve not only as electrical interconnections, but also acoustic reflecting surfaces for guiding and trapping the acoustic energy in the dielectric thin film. The acoustic cavity for the resonator is defined by the aluminum-silicon composite membrane structure. That membrane should be of low mass for high frequency operation, and that, in turn usually requires the removal of substrate material underlying the membrane portion of the thin film resonator. It has been typical to accomplish that by first forming a highly doped p.sup.+ region near the top surface of the semiconductor substrate, then etching a via from the bottom surface terminating at the p.sup.+ layer, which functions as an etchant stop. The thin film resonator is then formed on the p.sup.+ membrane. After formation of the thin film resonator, a selective etching process removes the p.sup.+ membrane, leaving the resonator suspended.
In this process of using a p.sup.+ membrane, several problems exist which contribute to the difficulty of manufacturing thin film resonators. For example, the p.sup.+ membranes are fragile and easily damaged. Moreover, the use of a p.sup.+ membrane causes misfit dislocations. These are inevitable with the diffusion of high dopant concentrations, and reduce the crystal integrity of the wafer surface for device manufacture.
Furthermore, the layers of the thin film resonator are themselves extremely thin and fragile. The requirement that the layers be deposited on a membrane which is itself extremely fragile, adds greatly to the difficulty and expense of manufacture and increases the likelihood of introducing manufacturing defects. Moreover, the creation of the p.sup.+ layer is costly and time consuming, requiring high-temperature equipment and long processing times.
Finally, the etchants used in the manufacturing process are extremely corrosive, especially to metals, and could not be permitted to come in contact with the thin film resonators. In the foregoing process, it was therefore necessary to remove the underlying substrate material with the etching step, and for the etching step to be completed prior to the deposition of the thin film resonator layers onto the substrate.